At subscriber's premises where more than one telephone may be offhook, such telephones may be of different types. Such different types of telephones may be capable of responding to SCWID information and others may not. Furthermore, some telephones may be capable of sending acknowledgement signals back to a telephone switch whereas others may not. In addition, some telephones are capable of extended service such as ADSI service and an indication must be provided to the switch as to whether or not an ADSI compliant telephone is connected to the subscriber line.
Telephone companies and the telephone industry have expressed a desire to always allow the highest level of service. This means that it is not acceptable to reduce to the lowest common denominator and not allow ADSI events to occur if multiple sets are offhook at the subscriber premises. In essence, it is desirable never to block ADSI events as this is a high value user pay service.
It would be desirable to send different acknowledgment signals, to indicate the presence or absence of offhook telephones of particular types on the subscriber loop.